I would like to apply an emission shader to specific parts of a texture
First of all you'll need a map that will define the areas where light is being emitted from, like this: 
Then to mask it over the diffuse map you'll need to create a simple node setup where you use the new texture to control the way two shaders mix, the white areas will use one shader, the dark ones the other one:



(You don't have to use a diffuse node, I just used it for the example) Your final result should be something like this:  
Obviously yours will be on the actual model. Hope this helps.